The designation of the electrodes in question as anode and cathode will here follow the convention that the electrode designated as anode is the one which, on discharge of the electrochemical storage device, brings about oxidation of the anode material. The respective other electrode will be designated as cathode. It is clear to a person skilled in the art in this respect that the function of the electrodes is reversed on charging of the electrochemical storage device.
The electrochemical storage device described and claimed here takes the form of a high-temperature storage device which requires a minimum temperature of at least 100° C. as its operating temperature. In particular, the operating temperature amounts to between 200° C. and 350° C. Under these temperature conditions, the operating temperature is sufficiently high for the solid electrolyte to have a good ion-conductive action. Good ion conduction consequently makes only a small contribution to the internal resistance of the electrochemical storage device.
However, it is also possible to provide an operating temperature of up to 500° C. Typical electrochemical storage devices to which the invention relates are those based on sodium-nickel chloride cell (NaNiCl2 cell) or sodium-iron chloride cell (NaFeCl2 cell) technology or mixed forms of said cells, or on sodium-sulfur cell (NaS cell) technology.
The structure of a conventional electrochemical storage device based on sodium-nickel chloride cell technology has a negative electrode which for instance takes the form of the anode during discharging operation, the anode material of which is liquid sodium at operating temperature. Said sodium typically fills part of the anode compartment. The positive electrode during discharging operation, i.e. the cathode, comprises a cathode compartment which is at least in part filled by a suitable metal, for instance nickel (Ni), mixed with a likewise suitable salt, for instance sodium chloride (NaCl), and for instance further suitable additives, for example aluminum chloride (AlCl3). At the operating temperature of the electrochemical storage device, the mixture typically at least in part takes the form of a liquid electrolyte.
It should also be noted at this point that the electrochemical storage device claimed according to the invention is here described as it is present in a typical working operating state. Since electrochemical storage devices sometimes have to undergo initial electrical charging prior to start-up, the configuration of the electrochemical storage device prior to start-up may sometimes be different from that described here. In particular, prior to initial charging the anode compartment may for instance not as yet have been filled with an anode material.
The anode compartment and cathode compartment are separated from one another by a solid electrolyte which, as a ceramic separator, is permeable only to ions. The solid electrolyte is not intended to permit any mass exchange other than that involving ions. Typical solid electrolytes which are used in sodium-nickel chloride cells comprise β-Al2O3 or β″-Al2O3 as the ceramic material. This permits a specific ion conductivity for Na+ ions which are transported between the anode compartment and cathode compartment through the solid electrolyte depending on the potential gradient. The crucial reactions during charging operation or discharging operation are revealed by the following reaction equation (discharging operation from left to right; charging operation from right to left): 2Na+NiCl22NaCl+Ni
At the equilibrium state, a voltage of approx. 2.58 volts may accordingly arise between the anode and cathode.
For practical applications such electrochemical storage devices are interconnected to form thermal modules, wherein metallically conductive contacting devices are typically mounted in the region of the top part for current tapping. The charge carriers here accumulate in the area surrounding these contacting devices, wherein a sometimes high local current density may however occur at these contacting devices. In the case of high power storage devices in particular, relatively significant heat production may arise in these regions due to the ohmic losses. However, this puts at risk reliable operation of such thermal modules and additionally brings about a reduction in the overall electrical efficiency of individual electrochemical storage devices of up to several percentage points in comparison with regular operation.
Such ohmic losses are accepted in storage devices which are not designed for high power applications, the losses being relatively low. However, if the electrochemical storage devices are exposed to very high local current densities and operated at the limits of their design, power losses of several watts may occur. In particular in the case of electrochemical storage devices based on sodium-nickel chloride cell technology, with walls sometimes made of relatively thin sheet metal, a voltage drop of up to over 50 mV may thus be caused, which corresponds for example at 100 A to a local heat source of around 5 watts of power per storage device.